As modern commerce depends on reliable and cost-effective methods for delivering products from suppliers to users, the availability of durable and reliable highways, roads and other support surfaces for vehicles is vital for sustaining a modern economy. To provide better support surfaces, highways, roads, and sidewalks are commonly paved with a layer or mat of asphalt concrete that is laid over the surface of the sub-base. Asphalt is preferred over Portland Concrete cement to paved roads because it is less expensive and very durable.
Asphalts are essentially mixtures of bitumen, as binder, with aggregate, in particular mineral filler, fine aggregates, and coarse aggregates. There are many different types of asphalts available and their characteristics can vary quite significantly. The design of asphalts for bituminous paving application is a complex process of selecting and proportioning materials to obtain the desired properties in the finished construction while minimize undesirable characteristics.
In evaluating and adjusting mix designs, the aggregate gradation and the binder content in the final mix design are balanced between the stability and durability requirements for the intended use. The final goal of mix design is to achieve a balance among all of the desired properties. Binders and various polymers have been investigated for reaching similar goals, and other modifications have been studied.
Unsaturated thermoplastic elastomers like styrene-butadiene-styrene (SBS) block copolymers, linear copolymers, diblock copolymers and radial copolymers are polymers used for asphalt modification. They enhance the elastic recovery capacities of asphalt and, therefore, its resistance to permanent deformations. However, unsaturated elastomeric polymers are quite expensive and are subjected to degradation when exposed to atmospheric agents and mechanical stress. This can result in a significant cost increase for the product. While SBS is recognized for performance benefits, research has focused on most cost effective modifiers in exchange for sacrificing superior performance.
Olefinic polymers have been investigated for use as modifiers. They are available in large quantities with different mechanical properties and at low cost. Polyethylene (PE) and polypropylene (PP) are plastomers. They bring a high rigidity (i.e., lack of elasticity, resistance to bending) to the product and significantly reduce deformations under traffic load. Due to their non-polar nature, PE and PP suffer from the drawback that they are almost completely immiscible with asphalt, and are thus limited in use.
Conventional asphalts often do not retain sufficient elasticity in use and, also, exhibit a plasticity range that is too narrow for use in many modern applications such as road construction. The characteristics of road asphalts can be improved by incorporating into them an elastomeric-type polymer. There exists a wide variety of polymers that can be mixed with asphalt. Of these, SBS is a commonly used polymer in asphalt modification. The modified asphalts thus obtained commonly are referred to variously as bitumen/polymer binders or asphalt/polymer mixes. There is a need for a modification to hot mix asphalt concrete mixes that would increase the resistance to permanent deformation while maintaining or decreasing the modulus of the mix at intermediate temperatures without affecting the binder properties significantly.
The bituminous binders, even of the bitumen/polymer type, which are employed at the present time in road applications often, do not have the optimum characteristics at low enough polymer concentrations to consistently meet the increasing structural and workability requirements imposed on roadway structures and their construction. In order to achieve a given level of modified asphalt performance, various polymers are added at some prescribed concentration. Current practice is to add the desired level of a single polymer, sometimes along with a reactant that promotes cross-linking of the polymer molecules for compatibility until the desired asphalt properties are met. This reactant typically is sulfur in a form suitable for reacting.
Numerous approaches have been used to incorporate sulfur as a crosslinking agent into asphalt compositions in the past. Typically the loadings are from about 0 wt. % to about 5 wt. % of the polymer. For non-crosslinking application, typically approaches have been to add between 5-70 wt. % sulfur into asphalt and non-asphalt based binders to perform in asphalt pavement applications.
When added to bitumen at 140° C., sulfur is finely dispersed in bitumen as uniformly small particles; coagulation and settlement of sulfur particles become noticeable after a few hours. Therefore, the sulfur extended asphalt (SEA) mixtures can be produced directly in the mixing plant just before the laying of the asphalt mixture. One major concern in handling sulfur-asphalt mix is the fear of the evolution of hydrogen sulfide (H2S) during production and laying. H2S evolution starts at temperatures higher than 150° C., so that the application at temperatures up to 150° C. avoids pollution and safety problems. However, H2S evolution starts well below 150° C., i.e. about 130° C. Moreover, below 120° C., neither the reaction of the asphalt and sulfur nor the cross-linking of the SBS/sulfur blend could take place.
A need exists for asphalt compositions that can be performance grade. It would be advantageous if the compositions could include materials, such as sulfur, that enhance performance of the compositions, while being relatively inexpensive.